To add to Patrickool93's advice, click not on the icon but on the text pane below it. Icon and name pane are separate entities for this purpose. First click highlights the name. Second click makes the text accessible for editing if the disk is not locked in Get Info. and if FileSharing is off (File::Sharing... from the menu).
If Sharing... is greyed-out, it is not active or not capable of being activated (because it hasn't been set up).
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